In a telephone communications system, the local loop is the connection between the customer premises and the switch in the local exchange. In the past, local loops were predominantly wired connections. Today, wireless local loops are increasing in popularity because of their wider bandwidths and increased flexibility.
To implement a communications system using wireless local loops, a multitude of wireless local loop base stations must be provided. Each base station services a predetermined number of customers in a given area. In one system, for example, each base station services 2000 customers. To use the system, each customer premises serviced by a particular local loop base station has to be fitted with a local loop antenna and transmit/receive circuitry to communicate with the base station. The local loop antenna would be mounted, for example, on an exterior wall of the customer premises and would be pointing in the general direction of the appropriate base station.
It is not inconceivable that a large percentage of the telephone users in the United States and around the world could someday be serviced by wireless local loops. This will require the production of millions of local loop antennas. Because the number of required antennas is so large, it is important that the antennas be relatively inexpensive to manufacture. That is, a small cost savings per antenna can add up to a very large savings by the time the millionth antenna is produced. Cost cutting, however, should not compromise the performance characteristics of the antenna or greatly reduce the structural integrity of the antenna.
Another consideration for local loop antennas, in general, is sidelobe suppression. Sidelobes are undesirable because they can cause interference with neighboring base stations or other transmit/receive equipment in the area. To achieve a given level of sidelobe suppression in an array antenna, amplitude tapering is generally employed. That is, the elements within the rows and/or columns of the array are driven at different excitation levels, with the excitation level at the center of a particular row or column being greater than the excitation levels toward the ends of the row or column. Such amplitude tapering reduces the sidelobe levels in a plane including the tapered row or column.
Theoretically, perfect sidelobe suppression can be achieved if an ideal binomial taper is used. An ideal binomial taper has an excitation profile that includes a peak center excitation level and geometrically decreasing side excitation levels that fall off by a factor of one-half for each successive element. For example, one such excitation profile is {a, 2a, 4a, 2a, a}. Non-ideal excitation profiles will produce sidelobe suppression of various degrees.
Because the size of a local loop antenna is normally limited, there is not always enough space to implement the number of elements required to achieve a desired level of sidelobe suppression. That is, an antenna may only be able to fit two side by side elements in a particular sidelobe plane, while three or more elements would be required to achieve a desired level of sidelobe suppression. It would be advantageous to be able to achieve a desired level of sidelobe suppression despite the limited number of elements in the plane of interest. In addition, amplitude tapering generally requires the use of unequal power splits to achieve the required excitation levels. These unequal power splits are difficult to implement and are generally lossy. It would be advantageous to develop a method for achieving a particular excitation profile without using unequal power splits.